1. Field of the Invention
The present invention relates to a clock signal switching device of an IC card. More particularly, the present invention relates to a clock signal switching device of an IC card for switching from an internal clock signal to an external clock signal supplied from a terminal unit. The IC card is operable in response to the internal clock signal and can be operated by the external clock signal in place of the internal clock signal when the IC card is attached to the terminal unit.
2. Description of the Background Art
IC cards provided with displays are utilized in various fields these days. Such an IC card is operable not only as a single body of a card but also in a form attachable to a terminal unit. For those purposes, an IC card contains a battery, an internal clock signal generating circuit for generating an internal clock signal, and a processing circuit. Further, in the form of the single body of the card, the processing circuit operates in response to the internal clock signal.
When such a card is attached to a terminal unit, an external power is supplied from the terminal unit to the IC card and an external clock signal is also supplied thereto. The processing circuit in the IC card operates in response to the external clock signal. Thus, switching between the clock signals is required to apply the internal clock signal when the IC card is to operate in the form of a single body and to apply the external clock signal when an IC card is attached to the terminal unit. For those purposes, it is necessary to determine whether the IC card is operating as a single body or it is connected to a terminal unit. Methods for such determination include a method of detecting a mechanical contact of an external terminal provided in the IC card, and a method of detecting a difference between the voltage of the battery contained in the IC card and that of the internal power supplied when the card is attached to the terminal unit.
However, in view of the structure of an IC card in general, it is difficult to attach a detector to its external terminal for the purpose of detecting the above mentioned mechanical contact. The method of detecting the difference of the voltages cannot always be utilized. For example, in cases in which there is only a small difference between the voltage of the battery contained in the IC card and the voltage of the external power supply, and when a noise is mixed in the case of using the IC card as a single body, it is often erroneously determined that the card is attached to a terminal unit, thereby causing errors in operation.
If an IC card is forcibly ejected from the terminal unit during communication of data between the card and the terminal unit, switching from the external clock signal to the internal clock signal occurs abruptly. This can cause disorder in the program contained in the IC card.